The hearts of 61 patients (age 64 plus or minus 11 years, 39 men) who died from 5 hours to 42 days (median 3 days) following a first fatal acute myocardial infarction (AMI) without having had percutaneous transluminal coronary angioplasty of coronary bypass surgery were studied to compare clinical and cardiac morphologic features of patients receiving thrombolytic therapy with tissue plasminogen activator (t-PA) to those not receiving thrombolytic therapy. Comparison of findings in the 23 patients who received t-PA intravenously 3 plus or minus hours after onset of symptoms to the 38 patients who did not showed: similar baseline characteristics with respect to: age, gender, history of hypertension; location of the AMI; heart weight; severity and numbers of coronary arteries narrowed; and frequencies of plaque rupture, plaque hemorrhage and coronary thrombi. Among the patients receiving t-PA, however, there was a greater frequency of platelet-rich (fibrin-poor) thrombi in the infarct-related coronary arteries (6 of 11 -vs- 4 of 25; p=.02), and a lower frequency of myocardial rupture (left ventricular free wall or ventricular septum) (5[22%] of 25 -vs- 18[46%] of 38; p=.045).